Artificial line network



. July 21, 1931- J. w. MILNoR Erm. 1,815,629

ARTIFICIAL LINI: NETWORK Filed sept. 4, 1929 4 sheets-sheet 1 July 21, 1931. 1w. MMOR m1. 1,815,629

I ARTIFICIAL LINE NETWORK Filed Sept. 4. 1929 4 Sheets-Sheet 3 Cullum July 21, 1931'. J. w. MILNOR ETAL ARTIFICIAL LINE NETWORK 4 sheets-sheet 4' Filed Sept. 4, 1929 @Som BN hm l? A TTORNE Y.-

d l Eik-lita 119311:

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PATENT OFFICE l" Josua w. MILNoiI. or MAiLEWooD, AND WILLIAM D. CANNON, or METUCHEN, NEW l JERSEY. AssmNons 'ro THE WESTERN UNION 'rameaux COMPANY, or NEW Yoan. N. Y.. A coBPonA'rIoN or NEW Yom:

ARTIFICIAL LINE NETWORK Application led September 4, 1829. Serial Ho. 390,348.

i This invention relates to submarine cable systems and to a method of and means for balancing a long submarine telegraph cable i by artificial line networks especially adapted l for use in such systems.

It relates particularly to the balancing of long high speed submarine cables for du lex operation, and especially to loaded ca les which 'employ non-loaded end sections for the purpose of reducing the severity of the balance requirements. However, the, methods and means of the invention are in general applicable to any'type of long cable either loaded or non-loaded.

Previously it has been practicable to successfully balance long submariney cables of the non-loaded type only. On these cables the attenuation' was so high that only moderate speeds were possible and a balance was.

required for relatively low frequencies only. It was ordinarily sufiicient here to balance the resistance and capacity by a fairly simple network; the small amount of inductance of the cable was approximately balanced by introducing certain irregularities in the resistance-capacity network, or by introducing a' few inductance coils placed near the head of the artificial line; or by the useof a special network such as shown in Patent No. 1,519,870. In some cases the inductance balance was omitted altogether.

Cables of recent construction howevenhave been designed to transmit signals at higher speeds. The higher speed necessitates a substantially perfect balance over a higher and a wider frequency range, and in a loaded cable this balance is complicated by the fact that the characteristics of the loading material are variable with frequency and current. Also certain other characteristics of the cable such as reflections, leakage, and dielectric absorption, which in the past could be largely neglected, assume considerable prominence under these conditions. An artificial line of the type described in the preceding paragraph may balance the terminal impedance of aum- Y absorption of the insulating material of the cable.

To reduce the effect of reflected currents on the receiving instrument. f Y,

. To Vbalance both the surge or characteristic Impedance and the propagation constant of the various types of cable which make up the completed cable.

The invention will be understood by reference to the accompanying figures in which: 1 represents the invention applied to Fig. Va ca le of the type disclosed in Patent 1,607 ,473 in which the main portion is loaded 'while the shore ends are left unloaded. This type of cable is referred to hereinafter as a composite cable. Y

Figs. 2, 3 and 4 illustrate forms of networks having characteristics suitable for balancing the seat return impedance of a submarine cable.

Fig. 5 shows a complete artificial line embodying the network of Fig. 2.

Fig. 6 shows a complete artificial line einbodyin the network of Fig. 3.

Fig. is a theoretical showing of a simple network for balancing the dielectric losses of a long submarine cable in which it is assumed that the capacity and leakage of the cable do not vary with frequency.

Fig. 8 shows an element of a network having a frequency variable characteristic, and

Fig. 9 shows a plurality of elementary networks like that of Fig. S connected in parallel with each other and also in parallel with a l larger fixed condenser, for simulating the dielectric properties of the submarine cable.

Figs. i0 and 11 show, in full line curves,

- the characteristics of the complete network including the three elementary networks in parallel to each other and to the large fixed condenser.

Figure 14 is a diagrammatic illustration of a complete cable similar to Figure 1, but embodyin the network arrangement disclosed in Figure 4p Fi re 15 shows the artificial line embodyin t e network Vof Figure 4.

eferring to Fig. 1, the long submarine cable is represented as being made up of a non-loaded cable section 1 at the shore en d and a loaded cable section 2. B thus re` movinor the loaded portion, which 1s the most diilicu t to balance, to a considerable distance from the terminal apparatus, a more accurate balance can be obtained. The cable is balanced for duplex operation, in accordance with the principles laid down in Patent No. 1,607 ,473, by an artiicial line divided into two portions, portion 3 for balancing the nonloaded cable 1 nearest the shore, and portion 4 for balancing the loaded cable 2.

The artificial line portion 3 comprises networks for balancing the conductor and sea return impedance, and the dielectric losses of the cable. The design of these networks, in the specific form shown, is based on the forms of networks shown in Figs. 3 and 9. The network for simulating the conductor and sea return impedance is made up of series inductances 10 and series resistance 11 with shunt paths 12 conductively connected about the series resistances 11, 11. The network for simulating the dielectric properties of the cable comprises shunt condensers 15, and additional networks 16 in shunt about some, but not necessarily all, of the shunt condensers 15. The additional networks 16, are made up of elementary networks each comprising resistance and capacity in series, the elementary networks being connected in parallel to each other and to thefixed shunt condenser 15.

Since portion 4 of the artificial line is designed to balance the loaded portion of the cable, the individual sections must necessarily contain a much larger amount of inductance. The shunting resistances are for, the purpose of simulating the eddy current losses in the loading material.y The sea return impedance is balanced by sections of the type shown in Fig. l3.

The terminal apparatus of the composite cable system of Fig. 1 is represented dia- -grammatically by a transmitter 20 connected to ground and to the junction of the bridge arms 21 and 22, the receiving apparatus 23.

being connected in usual manner across conjugate terminals of the bridge.

An understanding of the principles involved in designing the artiicial line of our invention may be had by consideration of the following:

The propagation constant may be written 1/( R +J`wL) (G +2005 and the surge impedance R -i-jwL G +jw C The expression Rid-jail; represents the linear resistance and reactance of the copper conductor together with the sea return path, and the expression represents the leakance and capacity reactance of the insulating material where,

R=resistance L=inductance G=1eakage C=capacity w =2 times the frequency In order that both the propagation constant and the surge impedance may be balanced in any given section or type of cable, it is essential that the values of shall be properly balanced in each section of cable. Each of the quantities R, L, G, and C, in themselves change somewhat with freuency. The manner in which G and C c ange is referred to as the dielectric absorpand tion. In artificial lines customarily used 1n has been found that this reactance can be balanced by a simple formof network which can be readily incorporated 'into the artificial line. This network'inv its simplest form is shown in Fig. 2. v

The manner in which the elements of this network may be determined soas to closely approximate kthe resistance and reactance ofthe conductor together with the sea return path of the cable, will be apparent from the following:

The impedance of the network oli-Fig. 2 using the symbols shown on the drawing, is:

Dividing this expression into its real and imaginary components, and dividing each-v by the length Z s o that the values apply to in which Rc and Xe designate `the real and imaginary parts of the conductor impedance.

Dilferentiating (2) andA (3) with respect to frequency; j

constants of the network, the values of Rc, Xe, A and B are determined from the-solid curves in Fig. 10er Fig. l1, the values being read at that frequency'which is most'important as determined from the operating'frequenc of the cable. These values are substitutedl in E nations 2, 3, 4 and 5, from which the values of f 1, R1, L2 and R2 are computed.

The dotted lines in Figs. 10 and 11 show the characteristics of equivalent networks chosen to simulate thel values of Re and Xe. The values of inductance and resistance of the network are given in the figures.. It will b e seen that the characteristics of the network "closely match those of the cable throughout the essential frequency range of the particular cable here chosen for illustrative pur poses,

The relative importance of the resistance and reactance' cfa non-loaded conductor is 10 and 11. Thus, the reactance is-of relatively small importance at the lower frequencies but becomes of increasing importance at the higher frequencies. That is, a

prolp'ortional to the height of the solid lines m `be much closer than the percentage accuracy of adjustment of the reactance.

Another network which is identical in characteristics but of more convenient form in' practice is shown in Fig. 3. The expression for its impedance is the same as that' for 2, when the following relationsV exist, using the symbols shown in the ligures:

Fig. 4 also is identical in characteristics under the following conditions:

Either of these three or other equivalent networks maybe used for the purpose as practical considerations dictate.

*The network of Fig. 2 when embodied in a 'complete artilicial line will have the form shown in Fig. 5, corresponding elements bearing the same designating characters. Thecondensers C1, G1, C1 simulate the capacity of the cable. .Y Y Y Similarly, the4 network of Fig. 3, when embodied in an actual artificial line, will have the form shown in Fig. 6. The net-- work of Fig. 4,.when embodied in an artilficial line, will have the form 'shown in Fig. l5. A complete cable terminal embodying this artificial line is shown in Fig. 14.

lt has been the custom to. use high grade mica or paper condensers for balancing the l capacity of long ocean cables. Although some small lossesare present in such condensers, they ordinarily comprise a moreperfect capacity than that which vis presented by the cable, and as a consequence they do not balance the cable with respect to certain loss characteristics of the cable dielectric. rihese characteristics are referred to as dielectric absorption. They have the nature of an absorption loss and are variable with frequency. For reasons previously stated, thesev characteristics should preferably be taken into account and should be balanced in the artificial line, order to obtain the best possible balance.' A

The method of simulating dielectric absorption in the artificial line is given below:

65 higher percentage accuracy of adjustment of If the capacity of the cable and the leak age did not vary with frequency, they could be represented by the simple network shown in Fig. 7 which embodies a pure capacity C and a leakance G in parallel to represent the losses.

Measurements of the dielectric of an actual cable treated as a network like Fig. 7 give a different value of C and G for eve frequenc the pure capa-city varying as s own by t e full-line curve of Fig. 12 while the leakance, expressed as a conductance varies according to the full-line curve of Fig. 13. It is desirable therefore to give to the artificial line capacity these same fre-l quency-variable characteristics. This cannot be accomplished bythe Fig. 7 network alone as the mathematical expressions for it lack-a frequencyl term.

Certaintypes'of simple network, however,

lhave been found to be variable with frequency. If the network of Fig. 8 is examined,

the expressions for its equivalent capacity and conductance arefound to be:

large, it is possible to obtain a vrough approximation to the leakage and capacity (i. e., the dielectric absorption) of an actual cable throughout the essential range, of fre- 5 quencies.

However, much better'- results will be obtained if about three networks similar t o Fig'. 8 are placed in parallel with each other and also in parallelwith -a lar er xedcondenser. The resulting networ is shown 1n Fig. 9. Each of the component networks will have characteristic e nations of the type shown iny Equations 8 an 9. The characteristics of the complete network may be ob tained simply by adding the characteristics of each component network. For each component network the product C X R should different from the others. In general, in practice the capacit C., may be 20 40 times as large as t e combined capacities 0f C1, C2, Cm

The results which may be obtained by such a network are shown in Figs. 12 and 13. In these curves-the dotted lines A, B and C respectively show the characteristics of the three networks Rl-Cl, Rif-C2, and Its-C3. The dotted lines marked Sum show the characteristics of the complete network including the condenser D. It lis. evident that this network gives a close approximation to the dielectric properties of the cable itself as indicated by the full lines.

If necessary, a still closer balance could be obtained by using four or more resistancecapacity networks instead of the three Lamaze The network shown has been found satisfactory for balancing dielectric. absorption, but itis to be understood that any one of numerous equivalent types lof network could .be used instead.

The resitance-capacity corrective branches do not need to be distributed -to the same extent as the main capacities C4. It is usually sufiicient to correct theY dielectric absorption in unit sections ranging from say 15 mile lengths at the head of the cable to lengths of several hundred miles some distance out. As the three branches operate independently of each other they can be distributed independently over the section as desired Each branch of the correction can be used either as a T-section or as a 1r-section.

Instead of the arrangement of series inductances 10 and series resistance 11 as shown in Fi 1, it is evident that these elements mayV interchanged, that is, the resistance 11 may be located at the head end of the artificlal line while the inductances may be at the earth end, as shown ,in Fig 14,-the network embodied in this figure being that of Fig. 4. In this figure the network for simulating the conductor 'and sea earth impedance is made up of seriesresistance 11 and series inductances 10, with shunt paths 12 conductively connected about the series inductances 10.

While thefeatures of this invention have been described with special reference to nonloaded cables .and to composite cables, it is clear that their applicability extends to completely loaded cables, the difference beingfonl;7 one of degree on the part of the inductance.

It is well known that when electric waves are incident upon discontinuities in a conductor, reflections of the waves occur. Small irregularities are frequently present in a cable due to differences in the manufacture of adjacent sections, or for other reasons. In constructing a balancing articial line similar irregularities are placed in the artiicial line at points corresponding to the locations of the irregularities as located in the actual cable. However, unless the section of cable between the irregularity and the trans mitting terminal is accurately matched in the artificial line the two reflected waves will differ slightly in magnitude and time of arrival so that the neutralization will not be I complete and a disturbing voltage will be impressed on the receiving instrument. ln the cable 'of Figure 1 the transition from the non-loaded to the loaded portion is accomplished by increasing the weight of loading at a very gradual rate but slight reflections are still liable to occur which at the high 'speeds and low current values used may injuriously aii'ect incoming signals.

However, with similar discontinuities accurately located in the artiicial line the refiection effects can be rendered negligible by balancing the-terminal section of the cable accordin to this invention. With the high degree o conjugacy which will obtain, such together with the variations with frequency ftaining the greatest" of each quantity, it isclear that the overall balance will be greatly-enhanced. With the aid of these refinements the duplex speed in each direction will be a maximum, thus obp'ossible messagecapacity of the cable.

We claim: Y

l. An artificial line adaptedto balance a submarine cable over a range voit-frequencies forming a signaling range, comprising a plurality of sections, each section including series impedance having resistance and inductive reactance, and shunt capacity, and a path containing inductanceand resistance conductively connected in shunt to a portion of said series impedance, the values of the impedance Y elements being so related thatboth the resistance and the reactance of said artificial line vary with frequency substantially in accordance with the variation with frequency of the corresponding constants of the submarine cable over the whole of the range of signal frequencies.

2. An artificial line adapted to balance a submarine cable over a range of frequencies' forming a signaling range, comprising a plurality of sections, each section including series impedances having resistance and inductive reactance, and shunt capacity, a lresistance path conductively connected in shunt to a portion of said series impedance., both the vresistance and the reactance of said network lbeing variable with frequency substantially 3. An artificial line adapted to balance a.

submarine cable over a range of frequencies vforming a signaling range, comprising a plurality of sections, each section comprising series impedances having resistance and inductive reactance, and shunt capacity, a resistive path conductively connected in shunt to -a portion of said series impedance, the resistance and the reactance of said network being variable withl frequency substantially in accordance with the variation with frequency of the corresponding constants of the submarine cable over the whole of said signaling usv range, and shunt paths about the capacities in a number of said sections, said shunt paths comprising resistance and capacity in series.

4. The, combination with a submarine cable having resistance and inductance,`the impedances of which vary over a signaling range, of a network for balancing the cable, said net-work having resistance and inductance whose impedances vary with frequency over said range, the resistance closely simulating that of the cable over substantially the whole of said range and the inductive reactance closely simulating that of the cable over at least the upper portion of the range.

5. The combination with a submarine cable having a sea return path whose impedance varies with frequency over a signaling range, of -a network for balancing the sea return impendance over said range comprisin series inductance and resistance elements an resistive paths in shunt to some of said elements, the impedances of said elements and resistive paths being so adjusted that the total resistance of the network closely simulates the resistance of the cable-and sea return path over ,substantially the whole of said range and the reactance of the network closely simulates the reactance of the sea return path over at least the upper portion of the signalin range.

6. ln com ina-tion, a submarine cable and an artificial line network arranged to balance the cable for duplex operation over a range of frequencies said network comprising means for simulating the propagationy constant of the cable and means for vsimulating the characteristic impedance of the cable sistive paths in parallel with some of said.

shunt elements.

7. An artificial line network designed to balance a conductor having resistance, inductive reactance, capacity and leakance that are variable with tre uency over a given range of frequencies orming a signaling range, which comprises a plurality of sections cach comprising series inductance and resistance and shunt capacity, a path in shunt to at least one of said series elements comprising inductance and resistance whose values are adjusted so that the combined resistance and reactance of the network simulates the corresponding constants of the conductor over the given range, and a path in shunt to at least one of said condensers comprising resistance and capacit whose values are adjusted so that the com ined capacity and leakance of the network simulate the corresponding constants of the conductor over the whole of said given range.

8. In combination, a composite cable having a central loaded portion and non-loaded end sections, and an artificial line associated with one end of said cable in balancing relation, said artificial line comprising a part designed to balancethe non-loaded portion of the cable and a part designed to balance the loaded portion of the cable, and means associated with said first mentioned part for balancing the sea return impedance through a range of fie uencies.

9. In combination, a composite cable having a central loaded portion and non-loaded end sections and an artificial line associated with one end of said cable in balancing relation, said artificial line coniprisiig a part designed to balance the non-loads portion of the cable and means associated with the first mentioned part for balancing both the characteristic impedance and the propagation constant. through a range of frequencies.

10. The method of increasin the accuracy and range of balance of a su marine cable balancing network which comprises balancing the capacity of the cable over a range of frequencies to a first approximation and successively introducing losses which vary with frequency t-ill the combined capacity and leakance of the network simulates the capacity and leakance of the cable over the said range of frequencies.

11. An artificial line for balancing the dielectric properties of a cable comprising condensers in spaced, parallel paths across the sections of the artificial line and means for causing the capacity and leakage of certain of said paths to vary with frequency in accordance with the variation with frequency of the capacity and leakage of the cable to be balanced, said means comprising resistance capacity shunts for a plurality of said condensers.

12. In an artificial line for balancing a cable, a network including a path having resistance and capacity in series and a parallel path comprising a capacity which is large with respect to said first mentioned capacity, the values of the resistance and capacity elements being proportioned so that the leakance and capacity of the network approximate the leakance and capacity of the cable over the range of frequencies for which the cable is balanced.

13. In an artificial line designed to balance the dielectric absorption of a cable, a network comprising a plurality of paths each having res1stance and capacity in series, and a parallel path'comprising a capacity which is large with respect to said first mentioned capacities, the product of capacity and resistance for each of the said plurality of paths being different.

14. `In combination, a composite cable having a central loaded portion and non-loaded end sections, and an artificial line associated with each end of said cable in balancing relation, each artificial line including a part designed to balance the adjacent non-loaded portion of the cable and a part designed to balance the central loaded portion, and means associated with said parts for balancing the sea return impedance of the cable.

15. The combination of a submarine cable having a return current path including inductance, and an artificial line in balancing relation therewith, said artificial line having means for simulating the impedance of the return path comprising inductance and resistance elements in series, one of said elements being shunted by an additional impedance element.

16. An artificial line network designed to balance a conductor having resistance, inductive reactance, capacity and leakance that are variable with frequency over a given range of frequencies forming a signaling range, which comprises a plurality of sections each comprising series inductance and resistance and shunt capacity, a path containing inductance and res1stance in shunt to at least a portion of one of said series elements and having values so adjusted that the combined resistance and reactance of the network simulates the corresponding constants of the conductor over the given range, and a path in shunt to at least one of said condensers comprising resistance and capacity whose values are adjusted so that the combined capacity and leakance of the network simulate the corresponding constants of the conductor over the whole of said given range.

17. An artificial line network designed to balance a conductor having resistance, inductive reactance, capacity. and leakance that are variable with frequency over a given range of frequencies forming a signaling range, which comprises a plurality of sections each comprising series inductance and resistance and shunt capacity, a path containing inductance and resistance in shunt to at least a portion of said series inductance, comprising inductance and resistance whose values are adjusted so that the combined resistance and reactaiice of the network simulates the corresponding constants of the conductor over the given range, and a path in shunt to at lgast one of said condensers comprising resistance and capacity/whose values are adjusted so that the combined capacity and leakance of the network simulate the corresponding constants of the conductor over the whole of said ngiven range.

' In testimony whereof we ax our signa- 5 tures.

JOSEPH W. MILNOR. WILLIAM D. CANNON. 

